The invention relates in general to semiconductor power field effect transistors (FETs), and more particularly to a structure and method for forming an improved inter-poly dielectric (IPD) in a shielded gate FET.
Shielded gate trench FETs are advantageous in that the shield electrode reduces the gate-drain capacitance (Cgd) and improves the breakdown voltage of the transistor. FIG. 1 is a simplified cross section view of a conventional shielded gate trench MOSFET. Trench 110 includes a shield electrode 114 directly below a gate electrode 122. Shield electrode 114 is insulated from adjacent silicon regions by a shield dielectric 112 which is generally thicker than gate dielectric 120. The gate and shield electrodes are insulated from one another by a dielectric layer 116 commonly referred to as inter-poly dielectric or IPD. The IPD layer must be of sufficient quality and thickness to support the required voltage between the gate and shield electrodes.
The conventional shielded gate FET of FIG. 1 suffers from a number of drawbacks. First, gate electrode 122 has sharp bottom corners which together with the flat top surface of shield electrode 114 leads to high electric fields in these regions. Second, conventional methods for forming the IPD typically introduce an oxide layer on the mesas between trenches that must be removed at some point after the gate electrode has been formed. In removing this oxide some etching of the gate oxide down the trench sidewalls inevitably occurs, which may result in gate shorts and gate leakage. Other known techniques tie formation of the IPD to formation of the gate dielectric and thus the IPD thickness is limited to a set multiple of the gate dielectric thickness. This does not allow independent optimization of the gate dielectric and the IPD. The largest differential in thickness between the IPD and the gate dielectric achieved has been about three to one (i.e., for a given target gate dielectric thickness, the largest IPD thickness that has been achieved is about three times greater than that of the target gate dielectric thickness).
Thus, there is a need for a structure and method of forming a shielded gate trench FET with improved IPD and gate dielectric.